A polymer electrolyte fuel cell comprises a plurality of single cells each comprising a polymer electrolyte membrane sandwiched between an anode and a cathode. The cells are stack one on another by an intervening electrode called a separator (or a bipolar plate).
The material used for the fuel cell separator is required to exhibit a low contact resistance and this low contact resistance should be maintained for a long time during its use as a separator of the fuel cell. In view of such requirement, use of a metal material such as an aluminum alloy, a stainless steel, a nickel alloy, and a titanium alloy has been contemplated also in view of workability and strength.
However, these materials have the problem that electroconductivity is greatly reduced by the oxide film formed on its surface when it is used for a fuel cell separator. As a consequence, despite its low initial contact resistance, such low contact resistance is not maintained for a prolonged period throughout its use as the separator, and the contact resistance increases with time to invite current loss. Another problem is deterioration of the electrolyte membrane by the metal ion dissolved out of the material by corrosion.
In view of such situation, a technique has been proposed to maintain the electroconductivity by suppressing increase in the contact resistance. For example, an electroconductive ceramic film may be formed on the metal surface to thereby suppress metal corrosion and maintain electroconductivity (see Patent Document 1).
Also proposed is a product produced by removing the passive film from the metal surface, covering the surface with a noble metal, for example, by plating to thereby maintain the electroconductivity, subjecting the metal to a compression treatment after covering the metal surface with the noble metal, and treating the metal in an active gas atmosphere for corrosion protection (see Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-Open No. 11-162479
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-105523